A study of chemoreceptor and baroreceptor A and C-fibres in the cat carotid nerve.

1. 149 A-fibres and 52 C-fibres from the cat carotid nerve were studied in vivo with single-unit recording techniques. These units subserved chemoreceptor and baroreceptor modalities. In addition, half of the C-fibres were determined to be efferent in origin. The estimated fibre diameter spectrum for chemoreceptor and baroreceptor A-fibres is described.2. The discharge pattern of chemoreceptor A and C-fibres was characteristically irregular both at rest and during activation. However, about 5% of the chemoreceptor A-fibre population exhibited a very regular discharge pattern, even at low rates of firing.3. In comparing A and C-fibres, it was found that chemoreceptor and baroreceptor A-fibres had lower thresholds, shorter response latencies, more rapid acceleration of discharge and higher discharge frequencies than their C-fibre counterparts.4. During strong chemoreceptor or baroreceptor stimulation, interaction of the ;spontaneous' whole nerve activity with the evoked A and C-fibre compound action potentials provided a method of estimating the relative proportions of chemoreceptors and baroreceptors in the A and C-fibre populations of the carotid nerve. The A-fibre population was found to be comprised of approximately 2/3 chemoreceptors, 1/3 baroreceptors. The reverse was true for the C-fibre population, i.e. 2/3 baroreceptors, 1/3 chemoreceptors.5. A stepwise C-fibre response is described which may arise from the several C-fibres within a single Schwann cell.